The present invention relates to an electrophotographic copier, and more particularly to a fixing arrangement provided for the electrophotographic copier especially for electrostatically transferring original images having large dimensions such as a plan or the like onto a copy paper which is successively fed from a continuous roll and transported through the copier without any interruption in feeding.
One of the most important requirements for a copier of the above described type is that such copier be provided with the capability of processing tracing paper the copy paper.
However, the tracing paper itself is apt to be curled or wrinkled in certain areas by sudden changes of the ambient temperature or humidity as compared with conventional copy paper. Therefore, even if an automatic feeder for ordinary copy paper operates well in electrophotographic copier, the tracing paper prepared in predetermined dimensions in advance, especially having large dimensions, can not be properly fed due to this deficiency.
Furthermore, the thermal fixing process which is specifically characteristic of the electrophotographic copying process often causes local wrinkles to take place in the tracing paper from the conventional load heating process used for fixing, and which wrinkles sometimes bring about the blocking or jamming of the paper fed to the fusing station of the electrophotographic copier.
Accordingly, to overcome disadvantages tracing paper in a roll form is conventionally utilized for an electrophotographic copier for copying original documents having large dimensions. The tracing paper is fed forwardly in a stretched condition and is passed through a series of image transferring steps, and successive image fixing steps, and finally out into required sheet lengths depending upon the dimensions of the original documents, to provide respective copies.
In the conventional copying process using a roll of tracing paper, the tracing paper to which the original image has been transferred is successively passed through the copier and the paper is rolled again, and hence, the tracing paper in the resultant roll is cut into respective sheets as the roll prepared in advance is used up. Therefore, since the copy paper in roll form is continuously stretched in the copier, the copy paper, even in the stand-by mode, is either subjected to the heater of the fixing device or urged towards a thermal fixing roll when the fixing device incorporated in the copier is a heating roll type device. In connection with the situation described above, one of the specific problems associated with the fixing device is that once the fixing device is heated up to a certain temperature level, it requires a certain period of time before the temperature of the fixing device attains an equilibrium with the surroundings even after the fixing device is de-energized. Therefore, there is a danger that the image bearing portion of the roll of tracing paper, which is normally kept stationary when copies are not being made, may be burned. Furthermore, as described in the foregoing, since the tracing paper is easily contracted or wrinkled even by a slight local temperature rise thereon, even the provision of conventional cooling means, for example, the forced convection type for cooling the fixing device, which cooling means may simply introduce a cooling medium by forced convection in and around the fixing device and the copy paper, can not properly prevent the temperature rise and consequent wrinkles described above.
To overcome the disadvantages described in the foregoing as well as to fuse the image on the copy paper or tracing paper in as short a time as possible, a variety of corrective measures have been proposed.
For example, according to "Heating Apparatus For Electrophotographic Copiers" disclosed in U.S. Pat. No. 3,922,520 patented Nov. 25, 1975, the fusing system comprises an active radiant energy source supported within a housing which may selectively be closed when in a stand-by mode. A portion of the housing also contains a passive heat source such as a suitable insulator material capable of storing large amounts of heat and then releasing such heat in the form of radiation when the housing is electrically actuated to be opened to the atmosphere.
In the conventional arrangment as described above, there is still a danger that the copy paper beneath the apparatus may be charred by the excessive heat radiated from the parts of the apparatus, especially immediately after continuous high speed copying is carried out.
Furthermore, according to the prior art described above, no precautionary measures are taken to prevent the occurrence of the damage, especially the burning of the copy paper, inside the apparatus, which may be especially brought about due to the presence of the passive heat arrangement described above when the electrical power supply is suddenly interrupted during the copying process.